1. Field of the Invention
The present invention relates in general to an apparatus for forming films on a substrate in a plasma.
2. Description of the Prior Art
Plasma vapor deposition apparatuses have been broadly used for forming films on substrates. Typical apparatuses of this kind comprises a reaction chamber, a pressure detecting device for detecting the pressure of the inside of the chamber, a pressure controlling device for controlling the pressure of the inside of the chamber, a pair of parallel plates between which a reaction or deposition region is defined, a gas feeding system for supplying a reactive gas, to the reaction space, a power supply for applying high frequency electric energy to the reaction gas at 13.56 MHz, a heater for heating a substrate to be coated, an evacuation system for evacuating the exhausted gas from the chamber. The parallel plates are a power supply electrode (cathode plate) connected to the power supply through a matching device, and an opposite electrode (anode plate) which is grounded.
The substrate to be coated is usually mounted on the opposite electrode grounded during plasma vapor deposition. In the case of forming films of certain materials or films having particular characteristics, e.g. carbon films having high hardnesses by means of the deposition chamber of this kind, the substrate to be coated is sometimes mounted on the power supply electrode in place of the opposite electrodes.
A self-bias potential is generated in the vicinity of the power supply electrode by virtue of the differences in mobility, mass and the like between different molecules, atoms, positive and negative ions, electrons, radicals and so forth occurring in the plasma. The ratio of the single bonds (C--C) to the double bonds (C.dbd.C) can be increased to form very hard carbon films by utilizing bombardment of positive ions on the growing carbon film in electric fields generated between the self-bias potential and the plasma potential.
The hardnesses of carbon films can be increased to the level of that of diamond also by decreasing hydrogen atoms coupled with carbon atoms to increase the proportion of the sp.sup.3 hybrid orbital bonds to the hybrid orbital bonds, e.g. sp or sp.sup.2.
Accordingly, it is important for forming hard carbon films to increase the above-mentioned self-bias potential. Typical and simple approaches, broadly employed in the field, for achieving this end are to decrease the pressure of the reactive gas and to increase the output power of the high frequency electric energy supplied to the reactive gas. However, the requirement of the deposition speed and reduction of the undesirable inner stress have to be compromised in this case.
The low deposition speed and the high inner stress are primary shortcomings of the plasma vapor deposition in forming carbon films. The shortcoming pose limitation on applications of this deposition technique for practical use. For example, the improvement of the deposition speed itself is possible in fact to several micrometers per minute when the quality of the film is not taken into account. When the respective parameters are optimized to achieve the qualities of deposited films, the deposition speeds are limited to 0.2 to 0.3 micrometers per minute in the case utilizing conventional hardware. The deposition speed can be improved by a factor of 1.5 by selecting appropriate reactive gases. However, this is the limit to the deposition speed at the present time.
FIG. 1 shows a conventional plasma vapor deposition apparatus. The apparatus comprises a reaction chamber 1, a pair of parallel plates 3 and 8 between which a reaction space is defined, an electrode shield 4, a gas feeding system 6 for supplying a reactive gas to the reaction space, a power supply 5 for applying high frequency electric energy to the reaction gas at 13.56 MHz, an evacuation system 7 for evacuating the exhausted gas from the chamber 1. The parallel plates are a power supply electrode 3 connected to the power supply 5 and an opposite electrode 8 grounded which are apart from each other by a relatively large distance of 20 to 50 mm, which distance is employed from the consideration of the construction of the apparatus, the stabilization of electric discharge, the uniformness of the film formed over the effective deposition area and so forth.
Because of this, the improvement of the deposition speed is achieved by the increase of the input power of high frequency energy, the increase of the pressure in the reaction space and so forth. It is, however, very difficult to increase the output power of high frequency energy since electric energy supplied can not effectively be transformed into the available high frequency electric energy in the usual wirings and the usual configuration of the electrodes, resulting in a substantial power loss. In addition to this, even if high power energy can be supplied to the reaction space, the influence of etching action prevails over that of the increase of the high frequency energy so that the deposition speed becomes sometimes reduced in spite of the increase of the high frequency energy.
The reduction of the inner stress is achieved only with a tradeoff for the increase of the hardness. Namely, when a harder film is formed, the residual inner stress is increased. In many practical cases, the advantage of the hardness is preferred to the disadvantage of the residual inner stress. In the cases where the thicknesses of films are increased to several micrometers, however, the residual inner stress poses serious problems of peeling-off, rubbing-off, decreased adhesivity and so on. A suitable technique is desired to form relatively thick films containing a decreased inner stress. With such difficult problems, very hard carbon films have been manufactured in return for low deposition speeds and high residual inner stress.